What exactly do you mean by "topped up"?
Membership, cash credit, data add-on?
Did mobile data work before now?
Are your APN settings configured correctly?
If not then text "APN" to 50048 and follow the instructions:
Do you have an active membership plan with a non-zero data balance?
What does your My48 account Dashboard say about your current membership status and expiration date and data balance?
What does your Transactions page say about recent payments?
Does your Buy Plan page list any upcoming memberships?
If you need to contact 48 support about this then see the link in my signature below or email them at support@48months.zendesk.com
